This invention relates to gas sensors, and, more particularly, to oxygen sensors.
Oxygen sensors are used in a variety of applications that require qualitative and quantitative analysis of gases. In automotive applications, the direct relationship between the oxygen concentration in the exhaust gas and the air-to-fuel ratio of the fuel mixture supplied to the engine allows the oxygen sensor to provide oxygen concentration measurements for determination of optimum combustion conditions, maximization of fuel economy, and the management of exhaust emissions.
A conventional stoichiometric oxygen sensor typically comprises an ionically conductive solid electrolyte material, a porous electrode on the exterior surface of the electrolyte exposed to the exhaust gases with a porous protective overcoat, and an electrode on the interior surface of the sensor exposed to a known oxygen partial pressure. Sensors typically used in automotive applications use a yttria stabilized zirconia based electrochemical galvanic cell with platinum electrodes, which operate in potentiometric mode to detect the relative amounts of oxygen present in the exhaust of an automobile engine. When opposite surfaces of this galvanic cell are exposed to different oxygen partial pressures, an electromotive force is developed between the electrodes on the opposite surfaces of the zirconia wall, according to the Nernst equation:   E  =            (                        R          ⁢                      xe2x80x83                    ⁢          T                          4          ⁢                      xe2x80x83                    ⁢          F                    )        ⁢          ln      ⁡              (                              P                          O              2                        ref                                P                          O              2                                      )            
where:
E=electromotive force
R=universal gas constant
F=Faraday constant
T=absolute temperature of the gas
PO2ref=oxygen partial pressure of the reference gas
PO2=oxygen partial pressure of the exhaust gas
Due to the large difference in oxygen partial pressure between fuel rich and fuel lean exhaust conditions, the electromotive force (emf) changes sharply at the stoichiometric point, giving rise to the characteristic switching behavior of these sensors. Consequently, these potentiometric oxygen sensors indicate qualitatively whether the engine is operating fuel-rich or fuel-lean, conditions without quantifying the actual air-to-fuel ratio of the exhaust mixture.
The internal resistance of a gas sensor significantly impacts the sensors performance. Areas affected include: light off time, steady state offset voltage, voltage output levels, and xe2x80x9cloading downxe2x80x9d effect of input impedance. The internal resistance of a gas sensor is comprised of three components: the linear electrolyte resistance, the nonlinear reference electrode polarization (overpotential), and the exhaust gas electrode polarization (overpotential). The first two components play a dominant role in the internal resistance, while the exhaust gas electrode polarization is not as important.
The linear electrolyte resistance and the nonlinear reference electrode polarization affect sensor performance because of the high electrical charge exchange rate with the electrolyte when platinum is used as the electrode. material. Because of this, the size of the electrodes, particularly the reference electrode plays an important role in determining the overall impedance of the sensor. Conventional reference electrodes are manufactured as large as the air reference chamber (as large as possible) due to the fear that the electrode would polarize due to diffusion limiting. Therefore, the impedance of the sensor would be large due to the small reference electrode.
Other sensor designs have attempted to lower the impedance of the sensor by having dual lower shields, a higher wattage heater, a lower mass element, or by reducing the zirconia thickness. However, although these methods reduce impedance, these processes are limited and tend to affect sensor performance.
What is needed in the art is an improved reference electrode that reduces impedance.
The deficiencies of the above-discussed prior art are overcome or alleviated by the gas sensor and method of producing the same.
In a preferred embodiment, a gas sensor comprises a first electrode and a reference electrode with an electrolyte disposed therebetween, wherein the first electrode and said reference electrode are in ionic communication, wherein the reference electrode has a surface on a side of the reference electrode opposite the electrolyte and the surface has a surface area. The gas sensor also comprises a reference gas channel in fluid conmmunication with the reference electrode, wherein at least a portion of the surface of the reference electrode physically contacts at least a portion of the reference gas channel, and wherein the portion of the reference electrode in physical contact with the reference gas channel is less than about 90% of the surface area.
In a preferred method, a gas sensor is formed by disposing an outer electrode and a reference electrode on opposite sides of an electrolyte such that the outer electrode and the reference electrode are in ionic communication, wherein the reference electrode has a surface on a side of the reference electrode opposite the electrolyte. Disposing at least a portion of a fugitive material in physical contact with a portion of the reference electrode surface, wherein the reference electrode has a surface area and the portion of the reference electrode surface in physical contact with the fugitive material is less than about 90% of the surface area. Disposing a heater on a side of the fugitive material opposite the reference electrode to form a green sensor and co-firing the green sensor.
The above-described and other features and advantages of the present invention will be appreciated and understood by those skilled in the art from the following detailed description, drawings, and appended claims.